1. Field of the Invention
The present invention relates to an image forming apparatus such as the electrophotographic copier and electrophotographic printer, and a process cartridge.
2. Description of the Related Art
A toner is used as a powder developer in the electrophotographic image forming apparatus such as a laser beam printer and copier using an electrophotographic method.
The toner is housed in a developer container, conveyed to a toner carrying member with a toner transfer mechanism, and carried on the toner carrying member. The toner is then endowed with a prescribed amount of electrostatic charge with a blade functioning as a toner layer thickness control member, and moves to an electrostatic latent image forming member on the image carrying member for carrying the image, visualizing the latent image on the photosensitive member there. The visible image is then transferred to a transcription member, such as a sheet of paper, with a transcription mechanism, and fixed with a fixing machine. The toner remaining on the image carrying member without being transferred to the transcription member is peeled off from the surface of the image carrying member with a cleaning device making contact with the image carrying member and is conveyed to a cleaning vessel. A series of image forming processes is completed as described above, enabling the user to obtain a desired image.
One development method known in the art includes a jumping development method in which the latent image on the image carrying member is developed while holding the developer carrying member of the image forming apparatus without making contact with the image carrying member. A development machine adapting the jumping development method will be described hereinafter.
The toner housed in the developer container is held on a development sleeve functioning as a developer carrying member in the development machine taking advantage of the jumping method. The toner as a one-component developer carried on the development sleeve is conveyed to a development area facing the photosensitive member as an image carrying member by rotating the development sleeve. The toner is controlled with a blade making the contact with the development sleeve during its transfer and is coated on the development sleeve forming a thin layer. The development sleeve is held at a distance of 50 to 500 .mu.m from the photosensitive member in the development area. The toner coated as a thin layer on the development sleeve jumps and adheres to the electrostatic latent image on the photosensitive member by applying a development bias voltage that is a superposition of direct current and alternating current from a bias power supply, thus visualizing the latent image as toner image.
The development bias voltage is also applied on a non-printing area such as the area between two sheets of paper.
However, it was difficult in the conventional development machine to obtain a desired image density because the amount of electrostatic charge given to the toner is so unstable that the increase in image density dulls the image at the initial stage when the image is continuously formed. Image defects are another problem under a high temperature and high humidity environment. Although these image defects may occur because of condensation on the surface of the photosensitive member, most of the image defects are caused by the following reasons. Talc contained in the transcription member adheres on the surface of the photosensitive member to allow oxides, formed by ozone generated from the electrostatic charging device, to react with moisture formed under a high humidity environment on the adhered talc, generating a product with ozone. Since this ozone product has low electrical resistance, the electrostatic charge in the charged potion on the image carrying member flows into the latent image portion to result in image defects. A decrease in durability due to the image defects was also an another problem.
Although image defects can be prevented from occurring when an abrasive is added in the toner to polish the surface of the image carrying member, irregular scrapes appear at minute portions on the image carrying member. When a part of the image carrying member has been deeply scraped, for example, the developer and additives accumulate there forming nuclei, where the toner damaged with the cleaning device may be sometimes fused.
Therefore, it was difficult in the conventional image forming apparatus to simultaneously solve all the problems of high resolution, fine image quality, durability and stability, image defects and fusion of the tone.